If the pressure was already low at startup when the engine was cold, I'd be leaning more toward a worn oil pump. It's hard to make a judgement call based on mileage because neither the pump nor the bearings should show excessive wear yet. The place to start is by insecting the crankshaft and connecting rod bearings. That's really not a very serious undertaking. It just requires draining the oil and removing the oil pan. If excessive clearance is found, technically the proper repair is to remove the crankshaft, have it machined and polished, and install matching under-size bearings. But removing the crankshaft involves removing the engine or transmission, so a lot of people just install new bearings with generally positive results. If simple bearing wear is the cause, oil pressure will be restored. If a bearing is "spun", meaning it has a rough, ground up surface, the crankshaft journal will also be severely damaged, and you'd be hearing banging noises too.
If bearing clearances are ok, put the new oil pump in. They are relatively inexpensive and provide cheap insurance. Be sure to insect the pickup screen for looseness.
Since the STP helped, go with a higher viscosity oil at the next oil change. You should not have to use an additive at every oil change as it only provides the same increased thickness the higher weight oil provides. You might also try a straight weight oil such as 30W or even 40W. These are not good choices for cold winter climates because they can cause hard starting due to low cranking speed, and it can put added stress on the shaft for shaft-driven oil pumps. I've only read about those shafts breaking a couple of times but why risk it?
Thursday, April 22nd, 2010 AT 2:22 PM